1. Field of the Invention
The present invention relates to an ON-period setting circuit, and to a power control IC and a switching power supply device incorporating it.
2. Description of Related Art
Compared with non-linear-control switching power supply devices (e.g., fixed-ON-period, fixed-OFF-period, and hysteresis-window switching power supply devices), linear-control switching power supply devices (e.g., voltage-mode-control and current-mode-control switching power supply devices) have the advantage of providing good load response with a simple circuit configuration.
Some switching power supply devices are furnished with a function of detecting reversal of a coil current under a light load to forcibly turn off a synchronous rectification transistor (a so-called reverse current cut-off function).
One example of the conventional technology discussed above is seen in U.S. Pat. No. 8,476,887.
Inconveniently, with conventional switching power supply devices, when a laminated ceramic capacitor with a low equivalent series resistance is used as an output smoothing capacitor, large output ripples result in a discontinuous-current mode (i.e., a light-load condition, where reversal of a coil current can occur).
Moreover, with conventional switching power supply devices, no hysteresis is given to the threshold value of the load current at which transition between a continuous-current mode (a heavy-load condition, where no reversal of a coil current occurs) and a discontinuous-current mode takes place. This makes transition between the two modes unstable, and causes repeated transition back and forth between the two modes, resulting in increased output ripples.